1897 LIGHT-SENSATIONS OF EYELESS ANIMALS. 175 



experimentally, by showing that if a number of oysters kept in a 

 vessel are found open, they will shut simultaneously the moment a 

 dark object comes between them and the light. While the shutting 

 of a single oyster on the passing of a shadow might be a mere coinci- 

 dence, the simultaneous clapping to of a number of individuals 

 excludes this possibility. 



The animal that gave the best reactions among the molluscs 

 was Psammohia vcspcrtina, a bivalve which, as the name implies, lives 

 in sand. The siphons of this animal protrude from the sand as 

 delicate, whitish, transparent tubes, without a particle of pigment. 

 If these siphons are suddenly illuminated, care being taken to avoid 

 any mechanical stimulation, they contract. It is important to note 

 however, that there is a short latent period of about a second. The 

 brighter the light, the greater the contraction. 



If a number of Psammobia are placed in a glass with plenty of 

 sand and carried into the direct sunlight, they bury themselves 

 instantaneously. If there is no sand in the glass, they flit about in 

 the water until exhausted. But while very bright light violently 

 stimulates these animals, it was observed that, with moderate illumi- 

 nation, it was not the light itself, but the sudden change from less to 

 greater which caused the reaction ; a moderate light appears to be 

 that to which they are accustomed. 



Having established the extreme sensitiveness of the siphons of 

 Psammobia to sudden increases of light, the experiments were varied. 

 Sudden diminutions were tried, but without other result than a slight 

 lateral movement. Psammobia therefore belongs to the group which 

 only responds to sudden increase of light, and this, it is found, 

 contrasts with another mixed group of animals which only respond to 

 sudden diminutions of light, and are not stimulated by its sudden 

 increase. These two different and opposite kinds of reaction are 

 called light-sensation and shadow-sensation. 



Again varying the experiments, Dr. Nagel found that illumination 

 with lights of different wave-lengths showed that all the visible 

 spectrum except the reds led to reaction. This effect is interesting 

 and important, and we shall have to refer to it again. 



It is not necessary to follow all the experiments, or to repeat a 

 list of the recorded cases referred to by the author. The sudden 

 closing of the sessile brachiopod Thecidium, on the passing of a shadow,, 

 is not given. I mention it here as a further indication of the secondary 

 acquirement of the dermatoptic function. This animal has a method 

 of life not unlike that of the oyster, although there can certainly be no 

 close genetic relationship between them. In both cases the animals 

 are exposed when the valves are open, and in both case? the lids clap 

 to on the passing of a shadow. 



A generalisation of interest to biologists must be noted in passing. 

 All the animals which responded to the sudden diminution of light 

 were such as inhabited strong shells, as in the cases just mentioned. 



